Snape as Head of Slytherin

[Littlered32773 at yahoo.com]

No: HPFGUIDX 30575

Does anyone find it odd that someone as young as Snape (early to mid 
30's according to interviews with JK, and the fact that he went to 
school with James), could be Head of Slytherin House?  It seems that 
the other HoH's are much older.  We know McGonagall is in her 70's 
(again interviews) and I don't see Sprout and Flitwick as much 
younger.  Given this, and Snape's  murkey past (yes, yes, Dumbledore 
vouched for him, but he WAS involved with Voldy as far as we know), 
doesn't it seem odd that he would be a HoH?  I get the impression 
from the books that he's been working at Hogwarts for several years 
before Harry arrives, though it couldn't be more than 10-12 (if they 
allow recent grads to work there immediately), so how did he move up 
to the prestigious HoH so quickly?